1. Field of the Invention
The present invention relates to a resin-coated carrier used in electrophotography in which a latent image formed on an image bearing member is developed into a visible image, a method of manufacturing the resin-coated carrier, a two-component developer containing the resin-coated carrier, a developing device using the two-component developer, and an image forming apparatus.
2. Description of the Related Art
Office automation (abbreviated as “OA”) equipments have been remarkably developed in these days and in line with such development, there has been a wide spread of copiers, printers, facsimile machines, and the like machines which form images through electrophotography.
For example, an image is formed by way of a charging step, an exposing step, a developing step, a transferring step, a fixing step, and a cleaning step in an image forming apparatus which employs electrophotography. At the charging step, a surface of a photoreceptor serving as an image bearing member is evenly charged in a dark place. At the exposing step, the charged photoreceptor receives signal light derived from a document image, resulting in removal of charges on the exposed part of the photoreceptor whose surface thus bears an electrostatic image (an electrostatic latent image). At the developing step, an electrostatic-image-developing toner (hereinafter simply referred to as “toner” unless otherwise mentioned) is supplied to the electrostatic image on the surface of the photoreceptor, thereby forming a toner image (a visualized image). At the transferring step, the toner image on the surface of the photoreceptor is transferred onto the recording medium by providing the recording medium with charges of which polarity is opposite to that of charges of the toner. At the fixing step, the toner image is fixed on the recording medium by means of heat, pressure, or the like. At the cleaning step, the toner is collected which has not been transferred onto the recording medium and thus remains on the surface of the photoreceptor. Through the above steps, a desired image is formed by the image forming apparatus employing electrophotography.
A usable developer for developing a toner image in the image forming apparatus employing electrophotography includes a one-component developer containing only a toner and a two-component developer containing toner and carrier. The two-component developer is provided with functions of stirring, conveying, and charging toner by the carrier. Accordingly, since toner in two-component developer does not need to have functions of carrier, the two-component developer has characteristics that the controllability is improved due to such separation of the functions, and a high-quality image is easily obtained, compared with one-component developer containing toner solely.
A carrier has two fundamental functions: the function of stably charging a toner to a desired charge level, and the function of conveying a toner to a photoreceptor. Furthermore, a carrier is stirred in a developing tank, and borne onto a magnet roller, on which the carrier forms a magnetic brush. Subsequently, the carrier passes through a regulating blade, and then returns to the inside of the developing tank. This allows the carrier to be reused. In continuing use of the carrier, the carrier is required to stably realize the fundamental functions, particularly the function of stably charge a toner. However, the carrier generally has large density and large stirring torque. Therefore, much driving power is required to stir the carrier in a developing tank.
In recent years, in view of environment, improvement in a carrier relating to low power consumption of an image forming apparatus is developed, and many investigations to decrease a density of the carrier are conducted to achieve low power consumption by reducing stirring torque of a developing tank. Furthermore, a carrier having low density tends to be investigated in the standpoint of long life of a carrier. To realize low density of a carrier, it is important to decrease density of a core material itself of the carrier.
Against such problems, Japanese Unexamined Patent Publication JP-A 2006-337579 and Japanese Unexamined Patent Publication JP-A 2007-57943 disclose a carrier in which a carrier core has voids and a resin is filled in the voids to achieve a low density, and the surface of the carrier core is coated with a silicone resin.
The amount of a resin which is used for coating the carrier core is generally about 2 parts by weight relative to the carrier core, however, the carrier disclosed in JP-A 2006-337579 and JP-A 2007-57943 requires at least 10 parts by weight or more, which is not realistic from the point of view of manufacturing. Specifically, the cost for manufacturing the carrier becomes high by increasing the amount of the resin to be used. Additionally, it is hard to control the thickness of a resin coating film for coating the surface of the carrier core filled with a resin due to a large amount of the resin to be used, and also when a resin capable of sufficiently impregnating the voids of the carrier core is added, carrier particles easily adhere to each other, and it is impossible to form a uniform resin coating film. In this manner, the resin-coated carrier in which thickness of a resin coating layer that is formed on the surface of the carrier core is non-uniform has a non-uniform abrasion ratio of the resin coating layer by stirring in a developing tank, and it is impossible to charge a toner stably.